Conventionally, cyclofluorocarbons (CFC), such as CC13F (CFC-11), have been used as a standard blowing agent for preparation of hard and soft polyurethane and isocyanate-based foams. However, since release of the substances to the atmosphere causes damage of an ozone layer in the stratosphere, use thereof has been inhibited. Accordingly, hydrogen-containing chlorofluoroalkanes (HCFCs) having a relatively short lifetime in the atmosphere, such as CHCl2CF3 (HCFC-123) and CH2ClCHClF (HCFC-141b), have been suggested as alternative blowing agents. However, HCFCs, which partially contain chlorine, also have relatively high Global Warming Potential (GWP) and Ozone Depletion Potential (ODP).
Accordingly, non-chlorinated, partially hydrogenated fluorocarbons (HFCs) have been suggested as blowing agent substituents for HCFC compounds. However, since HCFs have problems such as relatively high intrinsic thermal conductivity, i.e., low insulation properties, HFC-based blowing agents having improved insulation properties, such as CF3CH2CF2H (HFC-245fa), have been suggested. However, HFC-based blowing agents, such as HFC-245fa and recently developed HFC-134a and HFC-365 mfc, also have a disadvantage in that global warming indexes thereof are higher than a desired global warming index.
In addition, although hydrocarbon blowing agents, such as iso- and n-pentane, or cyclopentane, exhibit a very low global warming index, hydrocarbon blowing agents have a low heat insulation efficiency compared to, for example, HFC-245fa, as a blowing agent, and have combustibility.
The aforementioned blowing agents are used when polyurethane is foamed to provide a urethane foam layer. A resultant polyurethane foam is used as an insulation material between an exterior steel plate and a molded inner resin which are refrigerator components.
As a material for the molded inner resin, an acrylonitrile-butadiene-styrene resin (hereinafter referred to as “ABS resin”), which has superior processability, moldability, impact resistance, strength, gloss, and the like and thus is widely applied to various electric and electronic parts and accessories, may be used. Such an acrylonitrile-butadiene-styrene resin should have superior chemical resistance, which is not deteriorated by the aforementioned blowing agents, to prevent defect occurrence such as crack generation during production processes and use of products.
Accordingly, to improve chemical resistance of an ABS resin, various methods, such as a method of increasing the content of acrylonitrile included in the ABS resin, a method of increasing the molecular weight of the resin, a method of increasing a butadiene rubber content, a method of using a product including a rubber with a large average diameter, and a method of using an acrylate-based rubber, have been attempted. However, it has been difficult to provide a thermoplastic resin composition for a refrigerator inner case having a stricter level of chemical resistance while maintaining the same impact strength, gloss, vacuum formability, and the like as an existing ABS resin due to the development of a foaming agent having new eco-friendly characteristics and improved foaming efficiency.